Continuous extrusion of plastic products, such as tubing and plastic coated cables often suffers from the consequences of high frictional resistance between the extruded plastic product and the static walls of the extruder die. This is particularly true with regard to the extrusion of thermally cross-linkable materials, wherein cross-linking occurs in response to heat applied in the extruder die. This will be understood from the following description.
Plastics are divided generally into two main categories: thermoplastic materials and thermosetting materials. Thermoplastic materials consist mainly of macromolecules formed of long chains of monomers, based on carbon and/or silicon atoms, which may or may not be branched, hub are of finite size,
Molecules of thermosetting materials consist of three-dimensional networks which, theoretically, may extend infinitely, Thermoplastic materials may be formed either by cross-linking monomers or by cross-linking of thermoplastic materials. For example, Bakelite (R) (phenolformaldehyde) is formed generally by cross-linking of monomers, whereas cross-linked polyethylene is formed by cross-linking of thermoplastic polyethylene.
Thermosetting materials are preferable to thermoplastic materials in many respects, such as, heat resistance, mechanical strength, and low creep. By way of example, when polyethylene is cross-linked, its heat resistance, wear, creep characteristics, and resistance to chemicals are improved.
Notwithstanding the advantages of thermosetting materials, there is a certain difficulty in using them. This difficulty emanates from their particular internal structure which does not permit melting by heat of the material in its final form. It is difficult, therefore, to work these materials by use of some of the known processes.
It is especially difficult to use these materials in extrusion. This is due to the fact that thermosetting materials flows as a solid material, thereby causing very high frictional resistance between the extruder die wall and the extruded product. The consequences of such high frictional resistance during extrusion include reduced throughput, increased wear on the extrusion die and the extruded product and consequent decreased quality of the extruded product.
In order to overcome this problem, the prior art teaches extrusion of these materials prior to their being cross-linked, and in conditions which do not cause cross-linking. Cross-linking is thus carried out at a later stage, after extrusion, that is to say, after the extruded product has left the extruder die.
Throughout the specification and claims, the term "extrusion" is used to mean the forming process that is performed on the extruded material prior to its leaving the extruder die.
There are many methods of bringing about cross-linking after extrusion, in accordance with the material, technology and technical preference of the manufacturer. Some of these methods involve cross-linking immediately after extrusion, while others are performed at a later stage.
In the rubber industry, vulcanization (cross-linking) of natural thermoplastic rubber has been known for many years by the addition of sulfur and the application of heat.
It is also known to cross-link polyethylene, for example, by first, grafting of silane groups onto the monomer chain, prior to or during extrusion and, thereafter, exposing the extruded product to humidity, thereby to bring about cross-linking.
An alternative method of cross-linking of extruded polyethylene products is by exposing the extruded product to Beta or Gamma radiation.
Further methods include the preparation of suitable compounds which cause cross-linking on exposure to heat for a preselected time period. These methods involve extrusion at a relatively low temperature and/or for a short time, thereby preventing cross-linking during extrusion. The cross-linking is performed subsequently, by exposing the extruded product to heat at a higher temperature and for a preselected time. The heating may be provided, for example, by way of hot nitrogen, a hot salt bath, infrared heating, or microwave heating.
All of these prior art methods suffer from disadvantages, including the fact that they require an additional, separate process in order to provide the required cross-linking, thereby increasing both the time and cost of production. In many cases, the quality of the extruded products suffers from the effects of extruding at low temperatures and/or as a result of the addition of damaging cross-linking additives such as the above-mentioned silane groups.
Unsuccessful attempts have been made to carry out extrusion of thermosetting materials, in conditions that would allow cross-linking during the extrusion process.
One solution that is known in the art is to coat the interior surface of the extruder die with Teflon (R), thereby reducing the frictional resistance to flow of the thermosetting material.
It has, however, been found that the Teflon (R) wears very rapidly during the extrusion process, and it is, therefore, necessary to halt the production process frequently, in order to permit application of a new Teflon (R) layer to the die. These interruptions cause a loss in production time, both due to the actual coating of the die, and due to the additional start-up time that is required each time the process is restarted after coating. This loss in time, as well as the additional high cost of the Teflon (R) coating and its application, cause an unacceptably high increase in productions costs.